The present invention relates to a non-proteinaceous binder, coating system or coating composition especially adapted for the cast coating of paper, as described in U.S. Pat. Nos. 1,719,166 and 2,568,288 (Reissue No. Re. 23,637), these disclosures being incorporated herein by reference.
Casein, a proteinaceous material, has been the chief adhesive component in cast coating of paper since the origin of this process. However, casein has been the source of many price and availability fluctuations and has other disadvantages.
Attempts to replace the casein with synthetic latices have been unsuccessful, unrewarding or commercially impractical. Such prior attempts have been dependent upon employing an alkali-swellable or alkali-soluble polymer latex blended with an alkali-insoluble and non-alkali-swellable latex. More recently it has been proposed to maintain the compositions in which these latices are employed at acidic levels.
We have now found a particular non-proteinaceous binder for cast coating of paper comprising a modified starch and two styrene-butadiene latices of specific characteristics, which is highly suitable for its intended purpose. Our novel binder eliminates the need for casein and provides many advantages over heretofore known synthetically based compositions.
The cast coating of paper places rigorous demands and restraints on the type of coating binders that can be used successfully. One of the main requirements for a suitable binder in the cast coating operation is sustained drum release. In its simplest form the cast coating operation comprises applying an aqueous coating to a paper substrate. The coating is pressed against a rotating heated chromium surface. In the process the coating must stick and conform to the chromium surface while wet, but on drying must have auto-releasing properties.
The coating within the scope of this invention yields a commercial quality sheet of outstanding print quality, press performance, sheet gloss, water-resistance, fold, and freedom from curling of the paper. It also permits maximum drum speed in the coating operation.